This invention relates to Doppler radar navigation systems and, more particularly, to an improved transmit/receive antenna system for such a navigation system which is particularly well adapted for overwater use.
Antennas for overwater Doppler radar navigation systems must satisfy very stringent requirements. The type of antenna typically used for such an application is commonly referred to as a microstrip antenna and is formed as a planar printed circuit comprising an array of parallel lines of serially interconnected radiating rectangular patch elements. The antenna is mounted to the underbelly of an aircraft fuselage within a rectangular aperture formed by the ribs of the fuselage. Thus, the maximum size of the antenna is constrained by the spacing between the ribs. These Doppler antennas generate time shared beams within the defined aperture. Since beam width is inversely proportional to aperture size, one requirement is to utilize as much of the aperture as possible for each beam.
For Doppler systems that fly over both land and water, the navigation accuracy is impacted by a shift in the measured Doppler frequency due to the backscattering over water which is a function of the incidence angle (the angle from the vertical) and the actual sea state. The calmer the sea (the lower the sea state) the larger the Doppler error from land to sea because the sea has more of a mirror effect. It is therefore another requirement of such an antenna that it have the inherent ability to shape the beams so that they have contours which result in Doppler shifts which are essentially invariant with backscattering surface.
For FM/CW Doppler systems, the minimum required isolation between the transmit and receive antennas is sixty dB. This results in the requirement of space duplexed antennas (i.e., separate transmit and receive antennas). Since these antennas must both occupy the same aperture, this limits the full usage of the aperture for each of the antennas and conflicts with the requirement for narrow beam width.
Another requirement of such an antenna system is that it be inherently temperature and frequency compensated.
Planar microstrip antennas for Doppler radar navigation systems are well known. It is also known to slant the arrays in order to generate beams with particular contours to provide independence from overwater shift, as disclosed, for example, in U.S. Pat. No. 4,180,818, the contents of which are hereby incorporated by reference. U.S. Pat. No. 4,347,516, the contents of which are hereby incorporated by reference, discloses the application of the principles of the '818 patent to a rectangular antenna. However, the antenna according to the '516 patent only utilizes one half the available aperture for each of the beams. It is also known to interleave linear arrays so that the entire available aperture can be utilized for each beam and to use a crossover feed structure so that the antenna can be printed on only a single side of a substrate. Such structure is disclosed in U.S. Pat. No. 4,605,931, the contents of which are hereby incorporated by reference. However, the arrangement disclosed in the '931 patent provides all feeds from a single end of the antenna and only results in about half of the available aperture contributing to the shaping of each beam. When the width of an antenna employing the single-end feed scheme is reduced by half to accommodate a space duplexed configuration, the portion of the aperture contributing to beamshaping is also reduced by half. This reduced aperture is then unable to provide the degree of beam-shaping required for acceptable overwater performance.
It is therefore a primary object of the present invention to provide a transmit/receive antenna system satisfying all of the above requirements without the limitations of the known prior art.